During the past years, offshore drilling has continually increased. Given high costs and risks associated with offshore drilling, to avoid a dry well, marine seismic surveys are used to generate a profile (image) of the geophysical structure under the seafloor. While this profile does not necessarily provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas.
During a seismic exploration, as illustrated in FIG. 1, a vessel 10 tows seismic detectors 12 distributed along a cable 14. Cable 14 carrying seismic detectors 12 is called a streamer 16. A streamer may be up to ten km long and may be formed from plural sections of few hundreds of meters each. Vessel 10 may tow plural streamers 16 at once. Towed streamer 12 may have a constant depth relative to the ocean surface 18 throughout its length, or have a variable depth profile.
Vessel 10 also tows a seismic source 20 configured to generate seismic waves, which penetrate the solid structure under the seafloor 22, and are at least partially reflected by interfaces 24 or 26 between layers having different seismic wave propagation speeds. Seismic detectors 12 detect the reflected waves. The time delay between firing the seismic source and detecting a related reflection provides information about the location (depth) of reflecting interfaces.
Seismic exploration campaigns may last long periods (e.g., several months). During these campaigns, the streamers preferably remain in the water (except, for example, in cases of extreme unfavorable weather conditions) because recovering/deploying the streamers is expensive and tedious. Thus, the streamers are immersed in sea water for several consecutive months, at a shallow depth (a few meters), and are generally dragged at low speed (less than or equal to five knots). In these circumstances, the streamers' outer surfaces are subject to fouling, particularly due to the proliferation of microorganisms or bio-fouling. One of the most common types of microorganisms attaching to streamers is barnacles which adhere permanently to a hard substrate either by growing their shells directly onto the substrate or by means of a stalk. Examples of areas susceptible to marine grown on a streamer include birds, collars, weights and other places where higher levels of turbulence exist.
In time, bio-fouling results in substantial disadvantages such as: (1) generating hydrodynamic flow noise; (2) amalgams or incrustations forming on the streamer's outer surfaces, which are likely to disturb seismic measurements; (3) increasing drag on the streamers and, consequently, increasing fuel consumption to tow them; (4) a strong, unpleasant stench developing within a few days when streamers covered with bio-fouling are recovered and exposed to air; (5) bio-fouling attracts larger marine life which further increase the risk of noisy measurements and/or damage to the streamers; and (6) streamer skin being pierced by certain types of bio-fouling, resulting in liquid intake inside the streamer.
Several techniques have been applied conventionally to address the bio-fouling problem. For example, currently barnacle scraping can be performed by crews on work boats which increase a safety risk for workers. For another example, a cleaning device with rotary brushes and/or blades may be temporarily or permanently attached on streamers to clean their outer surfaces. The use of cleaning devices is impeded by the presence of protruding elements (e.g., floaters, or trajectory correction devices known as birds) on the streamers. Although cleaning devices continue to be subject to research and improvement, their practical value (reliability versus cost) remains questionable.
Another conventional technique which has been used to address the bio-fouling problem involves using antifouling paints (also used on boat hulls), for example, paints including cuprous oxides. Such antifouling paints are used with caution, from an environmental standpoint, to minimize sea water pollution. The safe use of antifouling paints may also be problematic for the persons likely to be in contact with antifouling paints during the manipulation of seismic streamers. Furthermore, techniques for applying antifouling paint are hardly compatible with technical and economic constraints linked to marine seismic streamers because painting them is a long and costly operation due to their length. In addition, to allow the paint to dry (to minimize seawater pollution), the painted seismic streamers have to be stored in a manner that requires a considerable amount of space, which is not economically viable onboard a vessel. Antifouling paint is more commonly currently used only on specific portions of a marine seismic streamer, e.g., birds, as the wear is quite high on the streamer during deployment and recovery that the paint life is undesirably short.
Another technique which has been used on small boats to reduce marine fouling is to place one or more transducers on the inside hull section of the boat. An example of such a system is the Ultra Series II Ultrasonic Antifouling system from UltraSonic Antifouling LTD. The transducers in this system then emit a low powered ultrasonic frequency which is pulsed and resonated through the hull. This creates an environment of moving water molecules over the underwater profile of the hull which in turn prevents, or possibly restricts, growth of microorganisms which can create fouling. However, it is unclear how to adapt such ultrasonic techniques from boat hulls to sensitive electronic equipment, such as marine seismic streamers.
Accordingly, it would be desirable to provide devices, systems and methods addressing the fouling problem related to streamers used in marine seismic explorations, in an economically attractive manner while avoiding the afore-described problems and drawbacks.